This invention comprises improvements to the methods and apparatus disclosed in my earlier U.S. Pat. No. 4,184,805 (January 1980) and U.S. Pat. No. 4,347,036 (August 1982) and, in general, is useful in Oscillating Cascade Power Systems (OCPS).
1. Field of the Invention
The present invention relates to techniques and apparatus for using the kinetic energy of a moving fluid stream and more particularly a cascade of aero- or hydro-foils oscillating in such a stream. These will produce useful work, e.g. generate electricity, by removing energy from the stream or produce propulsion by adding externally supplied energy to create a negative drag.
2. Description of the Prior Art
The continuing search for alternative sources of energy has caused a renewal of interest in utilizing the essentially inexhaustible kinetic energy of moving fluids such as wind, stream, river and ocean currents. These are derivative solar energy sources in that they are driven by solar heating. The windmill is a simple example of a device capable of harnessing such energy but is of limited utility because of the large centrifugal forces produced by large rotating blades, the dependence of the operating frequency on wind speed and the need for large areas of production, i.e. the circle made by the rotating blades, to produce sufficient power from naturally occurring wind flows.
One alternative to the windmill is disclosed in U.S. Pat. No. 4,024,409 to Peter R. Payne. This patent discloses a device including a wire which is oscillated by the shedding of vortices therefrom which oscillations are then converted into useful work. Like the aeolian harp, the Tacoma Narrows bridge and a street sign flapping in a gust of wind, this type of movement is induced when vortices are shed from a blunt body at a frequency in resonance with the natural frequency of the object. The patent also discloses the use of a single blade oscillated in response to wind conditions. However, as the windmill, the amount of energy which can be harnessed is limited. Furthermore, such oscillations are due to the Karman vortex street phenomena rather than the aeroelastic phenomenon of flutter.
U.S. Pat. No. 3,995,972 discloses a device including a stack of rigidly interconnected aerofoils positioned in a wind stream. By sequentially varying the angle of attack of the aerofoils, uniform oscillatory motion is produced for reciprocating a rod which then in turn drives an output device. The disadvantage of this system, like the windmill, is that there is no way to compensate for variations in wind speed to assure a substantially constant power output at a substantially uniform frequency.
Some work has also been done on the production of negative drag in the case of a single oscillating aerofoil, see I. E. Garrick, Propulsion of a Flapping and Oscillating Airfoil, NACA Rep. No. 567, May 1936.
It has long been known that a great amount of energy is available when an aerofoil is subjected to the phenomenon of aeroelastic flutter. While this phenomenon also applies to hydrofoils at sufficiently high water speeds, this was not observed in nature. Furthermore, studies of this phenomenon have been directed solely to preventing its occurrence because if left uncontrolled, it would lead to the eventual destruction of the aerofoil. My U.S. Pat. Nos. 4,347,036 and 4,184,805 addressed the useful application of this phenomenon for energy production and propulsion; these are extended here.